This invention relates in general to electrostatography and, more specifically, to apparatus and process for detecting subtle surface potential charge patterns on the outer surface of members.
In the art of xerography, a xerographic plate or photoreceptor comprising a photoconductive insulating layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990, the entire disclosure thereof being incorporated herein by reference. A photosensitive member is described in this patent having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photiogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
The photoreceptors are usually multilayered and comprise a substrate, an optional conductive layer (if the substrate is not itself conductive), an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer and, in some belt embodiments, an anti-curl backing layer.
Although excellent toner images may be obtained with multilayered photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, different imaging characteristics were encountered with photoreceptors fabricated of identical materials but in different fabrication runs. Since photoreceptor properties can vary from one production run to another, copy quality using photoreceptors from different production runs can be very different. This is because in the production of electrophotographic imaging members the complex nature of the manufacturing process renders unpredictable electrical characteristics of the coated photoreceptor from batch to batch and from month to month. For example, alteration of photoreceptor imaging properties can occur due to changes in the manufacturing environment, the installation or adjustment of new coating applicators or the initial use of a newly prepared batch of coating material for one of the many layers of the photoreceptors such as the hole blocking layer, charge generating layer, or charge transport layer. These changes in photoreceptor properties are difficult to identify within a reasonable length of time subsequent to the point in time when the photoreceptor comes off a production line.
One technique for detecting undesirable characteristics in photoreceptors from a specific production run is to actually cycle the photoreceptor in the specific type of copier, duplicator and printer machine for which the photoreceptor was fabricated. Generally, it has been found that actual machine testing provides an accurate way of detecting charge deficient spots in a photoreceptor from a given batch. However, machine testing for detecting nonuniform charging is a very laborious and time consuming process which requires involving hand feeding of sheets by test personnel along with constant monitoring of the final quality of every sheet. Moreover, accuracy of the test results depends a great deal upon interpretations and behavior of the personnel that are feeding and evaluating the sheets. Further, since machine characteristics vary from machine to machine for any given model or type, reliability of the final test results for any given machine model must factor in any peculiar quirks of that specific machine versus the characteristics of other machines of the same model or type. Because of machine complexity and variations from machine to machine, the data from a test in a single machine is not sufficiently credible to justify the scrapping of an entire production batch of photoreceptor material. Thus, tests are normally conducted in three or more machines. Since a given photoreceptor may be used in different kinds of machines such as copiers, duplicator and printers under markedly different operating conditions, detection of problem photoreceptors based on the machine tests of a representative test photoreceptor sample is specific to the actual machine in which photoreceptors from the tested batch will eventually be utilized will not necessarily predict whether the appearance of a problem will occur if the same type of photoreceptor were used in another different type of machine. Thus, for example, a machine charging uniformity test would have to be conducted on each different type of machine. This becomes extremely expensive and time consuming. Moreover, because of the length of time required for machine testing, the inventory of stockpiled photoreceptors waiting approval based on testing in machines can reach unacceptably high levels. For example, a batch may consist of many rolls with each roll yielding thousands of belts. Still further delays are experienced subsequent to satisfactory testing because the webs must thereafter be formed into belts, packaged and shipped.
Voltage variations which can occur on a charged imaging surface of a photoreceptor include charge deficient spots where the voltage sharply drops hundreds of volts along an extremely short lateral distance along the imaging surface of about 10 micrometers to about 100 micrometers. Typical two dimensional lateral sizes of charge deficient spots are on the order of between about 10 micrometers.times.10 micrometers and about 100 micrometers.times.100 micrometers. A line graph voltage profile of such a charge deficient spot has the appearance of a long narrow spike extending downwardly from the generally horizontal surface charge profile. Any probe for detecting a charge deficient spot must detect voltage changes over an extremely small area. The shield of the probe for detecting a charge deficient spot must be biased at about the average voltage of the photoreceptor surface to avoid air breakdown and destructive arcing. The resolution of this type of probe depends on the size of the probe electrode and the distance between the probe and the photoreceptor surface, typically very close to the photoreceptor, e.g. 100 micrometers. For measuring charge deficient spots, it is not necessary to have a very precise measurement of background voltage because the main function of background voltage is to bias the shield of a charge deficient measuring probe to prevent arcing.
Another type of voltage variation encountered is one which gradually fluctuates between about 1 to about 2 volts over a lateral distance along the imaging surface of between about 1 to 2 millimeters. A line graph voltage profile of such voltage variation appears as a rippled, substantially horizontal baseline voltage curve. This extremely subtle defect is particularly difficult to rapidly detect and cannot be detected by scanners designed for detecting charge deficient spots. This gradual voltage fluctuation nonuniformity is especially critical for advanced, highly sophisticated superimposed multiple image systems, particularly color imaging systems where at least one toner image is formed over at least one previously formed image and the resulting plurality of developed images are simultaneously transferred to a receiving member to produce full color prints or copies. For example, superimposed multiple electrostatographic image systems involve the formation of an electrostatic latent image on an imaging member, development of the latent image with toner particles to form a toner image, recharging and exposing of the charged imaging member to form a second electrostatographic latent image, and development of the second latent image with a toner having a color different from the previous toner. Additional charge, expose and development steps may be used for still additional colors. The final superimposed colored images are transferred to a receiving member. The photoreceptor coating thickness and deposited charge uniformity requirements for superimposed multiple image systems are extremely stringent and require highly sensitive electronic mapping of charge and thickness of the photoreceptor coating for detection of unacceptable imaging quality characteristics prior to distribution of photoreceptors to customers. The charging uniformity requirements for superimposed colored image systems is on the order of less than 5 volts. In other words, for testing of photoreceptors to be used in high tolerance imaging systems such as superimposed colored image systems, the electronic map of the charged surface of the photoreceptor should be accurate within 1 to 2 volts with a lateral resolution of less than about 1 millimeter. If the charging uniformity of a photoreceptor production batch fails to meet these stringent tolerance requirements, color shifting can occur during imaging which leads to copy quality degradation. Thus, there is a need for an improved system for mapping subtle changes in charge and coating thickness for photoreceptors.
Conventional charging devices such as wire and multi-pin electrode devices are inherently unstable and difficult to use in conducting, with any high degree of precision, scanning measurements for determining the uniformity of deposited charge and thickness of coatings on photoreceptors. Common examples of corona generating elements in a charging device in a xerographic copier or printer are thin wire, multiple pin, saw tooth blade electrodes. The geometry of a corotron is generally rectangular. The walls of the corotron serve as a shield. These corotrons extend across the width of belt photoreceptors or from one end to the other of drum photoreceptors and charge the passing imaging surface of a photoreceptor. It has been found that such corotrons do not produce uniform charging due to local nonuniformity of charging elements in the corotron or electrodynamic instabilities, for example, those exhibited by a saw tooth blade electrode. These electrode elements are also used in a scorotron which has grids of various shapes located at the charging end of the corona generating elements. The grid smoothes out nonuniformity of charging. However, the scorotron operates in a constant voltage mode and therefore covers up and hides nonuniformities in thickness and charge of photoreceptors. These charging devices perform satisfactorily for machines in which the required uniformity levels are of the order of 10 volts. However, these charging devices are unsatisfactory for highly sensitive electronic mapping of charge and thickness of the photoreceptor coating for detection of unacceptable imaging quality characteristics of photoreceptors for advanced, highly sophisticated superimposed colored image machines.